REVIEW 5 major objections 4 minor 1 cited by
Quadratic Programming-Based Posture Manipulation and Thrust-vectoring for Agile Dynamic Walking on Narrow Pathways
T0 review · 5 major / 4 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read A single convex controller lets a thrustered quadruped walk a 0.1 m beam and recover from a 40 N push.
desk verdict A standard centroidal-MPC simulation study with a serious state-space inconsistency in its linear model; the push-recovery ablation is solid, but the beam-walking claims need the fix and more baselines. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing object is the centroidal dynamics model with augmented thruster inputs: the robot's center-of-mass acceleration and angular acceleration are expressed as linear functions of four ground-reaction force vectors and four scalar thruster forces, with lever arms updated in real time from leg kinematics. Under the assumption of small roll and pitch angles, the attitude kinematics are linearized, yielding a convex model-predictive control problem whose constraints include a linearized friction cone and single-direction thrust limits. The optimizer selects both foot forces and thruster forces at 100 Hz, making the thrusters an active part of the stability loop rather than a separate assist.
What would settle it
Run the same ten-second beam-walking task and 40-newton push on the physical robot: if its attitude drifts, it leaves the beam, or it tips over while the simulator stayed upright, the sim-to-hardware transfer claim fails. A quicker in-simulation check is to initialize the walk with a roll angle outside the small-angle range, say 15 to 20 degrees, and see whether the linearized MPC still holds the beam; losing stability there would confirm the small-angle assumption as the effective limit.
Extended reading notes
Core claim
The central claim is that lateral stability on a narrow path can be achieved by adding thruster forces as inputs to a linearized centroidal dynamics model, then solving a short-horizon model-predictive control problem with ground-reaction-force friction-cone constraints and thrust bounds. The simulation shows the robot maintains a stable lateral position and stance height for ten seconds of beam walking, with thruster forces staying below 7 newtons, and recovers from a 40-newton lateral disturbance that causes the no-thruster controller to fall within two seconds. The authors conclude that thrusters effectively enlarge the feasible region of ground contact forces, so the no-slip condition can be met even when foot placement is severely restricted.
Load-bearing premise
The controller's prediction model assumes the robot's roll and pitch angles stay small, and the simulation assumes the physical robot's thrusters and ground contacts behave as modeled; if either fails in hardware, the demonstrated stability may not transfer.
Editorial extensions
If this is right
- The same MPC formulation covers both nominal beam walking and large lateral disturbance rejection, so no separate recovery-mode controller is needed.
- Because thruster forces on the beam stay below 7 newtons (about 7 percent of maximum thrust), only modest thrust authority is needed to stabilize roll dynamics in this scenario.
- Without thrusters the same controller fails within two seconds under the 40-newton push, showing the thrusters are what expand the recoverable disturbance envelope.
- The friction-cone constraints are satisfied for all four feet during beam walking, indicating the thrusters offload lateral ground forces enough to prevent slipping.
Reading between the lines
- If this transfers to hardware, the approach implies that trajectories previously classified as infeasible for legged robots due to narrow support areas become feasible once additional unilateral forces are available; the effective support polygon could be widened without widening the foot contact area.
- The paper's stated next target, walking on a flexible rope, would require extending the model from a rigid beam to time-varying contact geometry; the same convex MPC structure could still work if the reference states and contact point set are updated online.
- A testable extension is to measure the minimum beam width the controller can sustain as a function of maximum thrust; this would quantify how thrust authority trades off against required foot-placement precision.
- Because the thrusters act almost directly on the body's lateral dynamics, the framework may also apply to bipedal robots or to recovering from pushes on stairs, not just quadrupeds on beams.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper presents a simulation study of thruster-assisted narrow-beam walking for the Husky Beta quadruped. The authors formulate a centroidal dynamics model with ground reaction forces and propeller thrusts as inputs, linearize the attitude kinematics under a small-angle approximation, and build a model predictive controller that solves a quadratic program at 100 Hz. The paper reports two simulation experiments: a 40 N lateral push-recovery comparison with and without thrusters, and a 10 s walk on a 0.1 m wide beam. The claimed contribution is that a single convex MPC framework can coordinate foot forces and thrust forces to expand the locomotion envelope of a legged-aerial platform in simulation.
Significance. If the central claims held, the paper would provide a useful demonstration of convex MPC with thrust vectoring for multi-modal legged-aerial locomotion, and the push-recovery comparison is a well-posed control experiment. The authors should be credited for formulating the QP with friction-cone constraints, for using a high-fidelity PyBullet model, and for reporting thruster-force and friction-ratio trends. However, the current manuscript contains a state-vector inconsistency in the prediction model, unconstrained swing-foot forces in the QP, a missing no-thruster baseline for beam walking, and unreported controller parameters, so the evidence does not yet support the paper's central claim as written.
major comments (5)
- [Section III-A and IV, Eq. (7)] The state is defined as x = [θ^T, p^T, ˙ω^T, ˙p^T]^T, but the first block row of A multiplies the third state component by R_z^T. If the third component is ˙ω, the model predicts ˙θ = R_z^T ˙ω, which is dimensionally inconsistent with Eq. (5), ˙θ ≈ R_z^T ω. There is no ω state or integrator, so a nonzero angular velocity with zero angular acceleration would produce no orientation change in the prediction model. Because this prediction model is the core of the MPC, the state definition must be corrected to include ω (or the A matrix, Eq. (7), and the repeated definition in Section IV must be changed accordingly).
- [Section IV, Eq. (12)] The control input u includes u_{g,i} for all four legs and the B matrix in Eq. (7) includes all four ground-force columns, but the friction-cone constraints in Eq. (12) only bound u_{g,i} for i ∈ S_t. For swing legs, no constraint forces u_{g,i} = 0, so the QP can command nonzero ground reaction forces on feet that are not in contact, producing unphysical predicted dynamics and joint torques through Eq. (9). Please add equality constraints u_{g,i} = 0 for swing legs or remove the swing-leg ground-force columns from the prediction model.
- [Section V-C] The beam-walking experiment is only run with thrusters enabled. Because the paper's contribution is the thruster-assisted expansion of the locomotion envelope, a no-thruster baseline is needed to establish that the beam walking is enabled or improved by the thrusters. The push-recovery experiment provides such a baseline, but Section V-C does not, so the claim that the sagittal propeller played a significant role in satisfying the friction cone constraint is not supported by an ablation.
- [Section V-A and Eq. (12)] The reported simulation setup gives only the horizon (5) and the QP update rate (100 Hz). The state and control weights Q and R, friction coefficient μ_s, thruster limit u_max, stance time T_s, discretization step Δt, and PD gains are not reported. Without these values, the results are not reproducible and the reported friction-ratio and thrust-force margins cannot be interpreted quantitatively.
- [Section III-A and V-C] The small-angle linearization of Eq. (5) is load-bearing for the MPC prediction, but the paper does not report the actual roll and pitch excursions during the beam walk or the push recovery. The reader cannot verify that the robot remains in the regime where the approximation is valid. Please report the Euler-angle time histories or numerical maxima, and specify the quantitative bounds implied by the small-angle assumption.
minor comments (4)
- [Eq. (11)] The continuous-time gravity term h_g should be multiplied by Δt in the discretized expression, or A_k and B_k should be defined as exact discretization matrices. As written, x_{k+1} = A_k x_k + B_k u_k + h_g mixes continuous and discrete quantities.
- [After Eq. (7)] The phrase '0n and 1n donates' should read '0_n and 1_n denote'.
- [Eq. (5)] The phrase 'the robot never heads upward' is informal; please state the quantitative small-angle bounds used for the linearization.
- [Section V-B] The references to 'snapshots 2-5 in Figure 5(a)' and 'snapshots 6-8 in Figure 5(b)' are unclear because the multi-panel figure layout is not described; please label the snapshots and describe the panel contents in the caption.
Circularity Check
No significant circularity: the centroidal-dynamics MPC derivation is self-contained, and the simulation outcomes are not forced by the model construction; heavy self-citation is present but not load-bearing.
full rationale
The claimed contribution is a controller, not a derived empirical law, and the validation is simulation success. The model is built from Newton-Euler laws (Eqs. 1-3), linearized kinematics (Eq. 5, borrowed from the external MIT Cheetah 3 reference [29]), and a linearized centroidal prediction (Eqs. 7 and 11). The QP in Eq. 12 optimizes ground-reaction and thrust inputs subject to friction-cone and thrust limits; no parameter is identified from the successful trajectories, and no success metric is imposed as a constraint that would make the outcome definitional. References [6]-[17] are mostly prior work by the same group, but they are cited for hardware lineage and earlier thruster-assisted controllers, not as a uniqueness theorem or as a substitute for the model derivation; the approximation in Eq. 5 cites the external [29], and the foot-placement heuristic cites [30]. The skeptical dimensional inconsistency in the state vector (x contains \dot\omega, while the A matrix's first row maps that block to \dot\theta) is a genuine modeling and notation concern for correctness, but it is not circularity: it does not make the simulation result equal to the problem inputs by construction. Therefore no circular step is identified, and the score is low despite the heavy self-citation.
Assumptions & free parameters
free parameters (8)
- Q (state cost weighting matrix) =
Not reported
- R (control cost weighting matrix) =
Not reported
- mu_s (friction coefficient) =
Not reported
- u_max (thruster force limit) =
Not reported (plot implies 20 N in push recovery, 100 N max thruster)
- T_s (stance time) =
Not reported
- Dt (MPC discretization time step) =
0.01 s (implied by 100 Hz)
- n_h (prediction horizon) =
5
- PD gains Kp, Kd =
Not reported
assumptions (6)
- standard math Centroidal dynamics (Newton-Euler equations) govern the robot's overall translational and rotational motion.
- ad hoc to paper The small-angle approximation for roll and pitch holds during the maneuvers (Eq. 5), linearizing the attitude kinematics.
- domain assumption Foot-ground contact can be modeled with a linearized friction cone defined by a friction coefficient mu_s, and no-slip is ensured by satisfying these constraints.
- domain assumption Thruster forces act as external forces at the knee joint positions with known orientation e_i, and are limited to a single direction (positive thrust only).
- standard math The Raibert heuristic provides appropriate foot placement for the desired CoM velocity.
- domain assumption The PyBullet simulation with the high-fidelity model faithfully represents the real robot's dynamics, including thruster forces and ground contacts.
Cite this review
Pith. "Pith review of Quadratic Programming-Based Posture Manipulation and Thrust-vectoring for Agile Dynamic Walking on Narrow Pathways." pith.science (2026). https://pith.science/paper/M3WQX3H7
@misc{pith2026250723203,
author = {Pith},
title = {Pith review of: Quadratic Programming-Based Posture Manipulation and Thrust-vectoring for Agile Dynamic Walking on Narrow Pathways},
year = {2026},
howpublished = {\url{https://pith.science/paper/M3WQX3H7}},
note = {Machine review of arXiv:2507.23203}
}
abstract
There has been significant advancement in legged robot's agility where they can show impressive acrobatic maneuvers, such as parkour. These maneuvers rely heavily on posture manipulation. To expand the stability and locomotion plasticity, we use the multi-modal ability in our legged-aerial platform, the Husky Beta, to perform thruster-assisted walking. This robot has thrusters on each of its sagittal knee joints which can be used to stabilize its frontal dynamic as it walks. In this work, we perform a simulation study of quadruped narrow-path walking with Husky $\beta$, where the robot will utilize its thrusters to stably walk on a narrow path. The controller is designed based on a centroidal dynamics model with thruster and foot ground contact forces as inputs. These inputs are regulated using a QP solver to be used in a model predictive control framework. In addition to narrow-path walking, we also perform a lateral push-recovery simulation to study how the thrusters can be used to stabilize the frontal dynamics.
Figures
Figures from the paper (5 more)
Forward citations
Cited by 1 Pith paper
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Analysis of Harpy's Constrained Trotting and Jumping Maneuver
The provided manuscript text does not contain the claimed analysis of the Harpy robot, making the abstract's conclusions unverifiable from this document.
Reference graph
Works this paper leans on
- [1]
-
[2]
Hoatzin nestling locomotion: Acquisition of quadrupedal limb coordination in birds,
A. Abourachid, A. Herrel, T. Decamps, et al. , “Hoatzin nestling locomotion: Acquisition of quadrupedal limb coordination in birds,” Science Advances, vol. 5, no. 5, eaat0787, May 2019
work page 2019
-
[3]
Bipedal locomotion: Effects of speed, size and limb posture in birds and humans,
S. M. Gatesy and A. A. Biewener, “Bipedal locomotion: Effects of speed, size and limb posture in birds and humans,” Journal of Zoology, vol. 224, no. 1, pp. 127–147, 1991
work page 1991
-
[4]
E. Sihite, A. Kalantari, R. Nemovi, A. Ramezani, and M. Gharib, “Multi-Modal Mobility Morphobot (M4) with appendage repurpos- ing for locomotion plasticity enhancement,” Nature Communications, vol. 14, no. 1, p. 3323, Jun. 2023
work page 2023
-
[5]
Wing-Assisted Incline Running and the Evolution of Flight,
K. P. Dial, “Wing-Assisted Incline Running and the Evolution of Flight,” Science, vol. 299, no. 5605, pp. 402–404, Jan. 2003
work page 2003
-
[6]
Generative Design of NU’s Husky Carbon, A Morpho-Functional, Legged Robot,
A. Ramezani, P. Dangol, E. Sihite, A. Lessieur, and P. Kelly, “Generative Design of NU’s Husky Carbon, A Morpho-Functional, Legged Robot,” in 2021 IEEE International Conference on Robotics and Automation (ICRA) , May 2021, pp. 4040–4046
work page 2021
-
[7]
Rough-Terrain Locomotion and Unilateral Contact Force Regula- tions With a Multi-Modal Legged Robot,
K. Liang, E. Sihite, P. Dangol, A. Lessieur, and A. Ramezani, “Rough-Terrain Locomotion and Unilateral Contact Force Regula- tions With a Multi-Modal Legged Robot,” in 2021 American Control Conference (ACC), May 2021, pp. 1762–1769
work page 2021
-
[8]
Unilateral Ground Contact Force Regulations in Thruster-Assisted Legged Locomotion,
E. Sihite, P. Dangol, and A. Ramezani, “Unilateral Ground Contact Force Regulations in Thruster-Assisted Legged Locomotion,” in 2021 IEEE/ASME International Conference on Advanced Intelligent Mechatronics (AIM), Jul. 2021, pp. 389–395
work page 2021
Show all 31 references
-
[9]
Optimization-free Ground Contact Force Constraint Satisfaction in Quadrupedal Locomotion,
E. Sihite, P. Dangol, and A. Ramezani, “Optimization-free Ground Contact Force Constraint Satisfaction in Quadrupedal Locomotion,” in 2021 60th IEEE Conference on Decision and Control (CDC), Dec. 2021, pp. 713–719
2021
-
[10]
Salagame, M
A. Salagame, M. Gianello, C. Wang, et al. , Quadrupedal Loco- motion Control On Inclined Surfaces Using Collocation Method , arXiv:2312.08621 [cs, eess], Dec. 2023. DOI: 10.48550/arXiv. 2312.08621. [Online]. Available: http://arxiv.org/abs/ 2312.08621 (visited on 07/08/2024)
-
[11]
Control of Thruster-Assisted, Bipedal Legged Locomotion of the Harpy Robot,
P. Dangol, E. Sihite, and A. Ramezani, “Control of Thruster-Assisted, Bipedal Legged Locomotion of the Harpy Robot,” Frontiers in Robotics and AI , vol. 8, 2021
2021
-
[12]
A HZD-based Framework for the Real-time, Optimization-free Enforcement of Gait Feasibility Constraints,
P. Dangol, A. Lessieur, E. Sihite, and A. Ramezani, “A HZD-based Framework for the Real-time, Optimization-free Enforcement of Gait Feasibility Constraints,” in 2020 IEEE-RAS 20th International Con- ference on Humanoid Robots (Humanoids) , Jul. 2021, pp. 156–162
2020
-
[13]
Performance satisfaction in Midget, a thruster-assisted bipedal robot,
P. Dangol, A. Ramezani, and N. Jalili, “Performance satisfaction in Midget, a thruster-assisted bipedal robot,” in 2020 American Control Conference (ACC), Jul. 2020, pp. 3217–3223
2020
-
[14]
Demonstrating Autonomous 3D Path Planning on a Novel Scalable UGV-UA V Morphing Robot,
E. Sihite, F. Slezak, I. Mandralis, et al., “Demonstrating Autonomous 3D Path Planning on a Novel Scalable UGV-UA V Morphing Robot,” in 2023 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS) , Oct. 2023, pp. 3064–3069
2023
-
[15]
Capture Point Control in Thruster-Assisted Bipedal Locomotion,
S. Pitroda, A. Bondada, K. Venkatesh, et al., “Capture Point Control in Thruster-Assisted Bipedal Locomotion,” in 2024 IEEE Interna- tional Conference on Advanced Intelligent Mechatronics (AIM) , Jul. 2024, pp. 1139–1144
2024
-
[16]
Pitroda, E
S. Pitroda, E. Sihite, T. Liu, et al., Enhanced Capture Point Control Using Thruster Dynamics and QP-Based Optimization for Harpy , arXiv:2411.17727 [cs], Nov. 2024. DOI: 10 . 48550 / arXiv . 2411.17727. [Online]. Available: http://arxiv.org/abs/ 2411.17727 (visited on 03/12/2025)
-
[17]
Pitroda, E
S. Pitroda, E. Sihite, K. V . Krishnamurthy, et al. , Quadratic Pro- gramming Optimization for Bio-Inspired Thruster-Assisted Bipedal Locomotion on Inclined Slopes , arXiv:2411.12968 [cs], Nov. 2024. DOI: 10 . 48550 / arXiv . 2411 . 12968. [Online]. Available: http : / / arxiv...
-
[18]
OSQP: An Operator Splitting Solver for Quadratic Programs,
B. Stellato, G. Banjac, P. Goulart, A. Bemporad, and S. Boyd, “OSQP: An Operator Splitting Solver for Quadratic Programs,” Mathematical Programming Computation , vol. 12, no. 4, pp. 637– 672, Dec. 2020
2020
-
[19]
qpSWIFT: A Real-Time Sparse Quadratic Program Solver for Robotic Applications,
A. G. Pandala, Y . Ding, and H.-W. Park, “qpSWIFT: A Real-Time Sparse Quadratic Program Solver for Robotic Applications,” IEEE Robotics and Automation Letters , vol. 4, no. 4, pp. 3355–3362, Oct. 2019
2019
-
[20]
Parallelizing the dual revised simplex method,
Q. Huangfu and J. A. J. Hall, “Parallelizing the dual revised simplex method,” Mathematical Programming Computation , vol. 10, no. 1, pp. 119–142, Mar. 2018
2018
-
[21]
ECOS: An SOCP solver for embedded systems,
A. Domahidi, E. Chu, and S. Boyd, “ECOS: An SOCP solver for embedded systems,” in 2013 European Control Conference (ECC) , Jul. 2013, pp. 3071–3076
2013
-
[22]
O’Donoghue, Operator splitting for a homogeneous embedding of the linear complementarity problem , en, Apr
B. O’Donoghue, Operator splitting for a homogeneous embedding of the linear complementarity problem , en, Apr. 2020. [Online]. Available: https : / / arxiv . org / abs / 2004 . 02177v4 (visited on 04/12/2024)
2020
-
[23]
Bambade, F
A. Bambade, F. Schramm, S. El-Kazdadi, S. Caron, A. Taylor, and J. Carpentier, PROXQP: an Efficient and Versatile Quadratic Pro- gramming Solver for Real-Time Robotics Applications and Beyond . Sep. 2023
2023
-
[24]
qpOASES: A parametric active-set algorithm for quadratic pro- gramming,
H. J. Ferreau, C. Kirches, A. Potschka, H. G. Bock, and M. Diehl, “qpOASES: A parametric active-set algorithm for quadratic pro- gramming,” Mathematical Programming Computation, vol. 6, no. 4, pp. 327–363, Dec. 2014
2014
-
[25]
Caron, A
S. Caron, A. Zaki, P. Otta, D. Arnstr ¨om, J. Carpentier, and F. Yang, qpbenchmark: Benchmark for quadratic programming solvers available in Python , version 2.2.1, Feb. 2024. [Online]. Available: https://github.com/qpsolvers/qpbenchmark
2024
-
[26]
com / quadprog / quadprog, Version 0.1.11, 2021
quadprog developers, Quadprog: Quadratic programming solver (python), https : / / github . com / quadprog / quadprog, Version 0.1.11, 2021
2021
-
[27]
Legged Walking on Inclined Surfaces,
C. Wang, “Legged Walking on Inclined Surfaces,” English, ISBN: 9798379484637, M.S. thesis, Northeastern University, United States – Massachusetts, 2023. [Online]. Available: https : / / www . proquest . com / docview / 2808490798 / abstract / C77A6E9C9EDB4958PQ/1 (visited on 0...
2023
-
[28]
The pinocchio c++ library – a fast and flexible implementation of rigid body dynamics algorithms and their analytical derivatives,
J. Carpentier, G. Saurel, G. Buondonno, et al., “The pinocchio c++ library – a fast and flexible implementation of rigid body dynamics algorithms and their analytical derivatives,” in IEEE International Symposium on System Integrations (SII) , 2019
2019
-
[29]
Dynamic Locomotion in the MIT Cheetah 3 Through Convex Model-Predictive Control,
J. Di Carlo, P. M. Wensing, B. Katz, G. Bledt, and S. Kim, “Dynamic Locomotion in the MIT Cheetah 3 Through Convex Model-Predictive Control,” in 2018 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS) , Oct. 2018, pp. 1–9
2018
-
[30]
M. H. Raibert, Legged robots that balance . MIT press, 1986
1986
-
[31]
Coumans and Y
E. Coumans and Y . Bai, Pybullet, a python module for physics simulation for games, robotics and machine learning , 2016
2016
Reviewed August 6, 2026 · model on record in the stance chip above.
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